Various arrangements for determining the rotational speed of a transmission shaft are known from the state of the art. Also known as rotational speed detection devices, these arrangements for generating a speed signal for a motor vehicle or the like essentially comprise a signal or rotational speed transmitter rotationally fixed to the corresponding transmission shaft. A stationary rotational speed sensor is associated with the rotational speed transmitter in order to detect the rotational speed of the rotational speed transmitter.
In general, this is the way in which DE 198 14 758 A1 describes a device for detecting the rotational speed of a shaft. The known device features a signal transmitter that is rotationally fixed to the shaft. The signal transmitter is configured like a gear wheel, where the teeth of the gear wheel each feature a steep and a flat tooth flank. In addition, an analog Hall effect sensor is provided that detects the rotational speed in a radial direction on the periphery of the gear wheel, i.e., on the teeth. Thanks to the different angles of the two flanks of each tooth, it is possible to determine the rotational speed and the direction of rotation of the shaft with only one sensor and at a constantly updating rate, even when the shaft is rotating at a very low rotational speed.
DE 103 47 494 A1 describes a similar arrangement that is supposed to determine the torque of transmission shafts. For this purpose, the known device is comprised of a transmission shaft on which a rotational speed transmitter gear wheel is rotationally fixed, where a stationary rotational speed sensor is once again associated with the rotational speed transmitter gear wheel. The teeth of the rotational speed transmitter gear wheel, as they rotate past the rotational speed sensor, produce electric impulses, which are fed to a corresponding evaluation unit.
The state of the art shown above has proved its value in practice, but the additional signal transmitter, rotational speed transmitter or rotational speed transmitter gear wheels result in an increase in the total length of the transmission and to an increase in weight, which can be a disadvantage with certain arrangements of the transmission inside the motor vehicle.
In comparison, DE 102 23 625 A1 proposes a rotational speed detection device for a transmission of a motor vehicle that likewise comprises a signal transmitter and a sensor, where the signal transmitter is integrated in a dust-proof cover for an output-side, shaft-sealing ring. The dust-proof cover is arranged outside the transmission housing and is connected in a rotationally fixed manner to the housing. This solution is an advantage in so far as the existing dust-proof cover functions at the same time as a signal transmitter, which means that the weight of the transmission is not increased by an additional component. Nevertheless, the externally situated sensor leads to an enlargement of the transmission, which makes installation in a motor vehicle more difficult. Furthermore, it is necessary to accordingly modify the cover to protect against dust so that it can function as a signal transmitter. It is proposed that equidistant size increases be provided on the periphery of the dust cover which, not only further increase the size of the transmission, but which also result in greater fabrication expense.
It is the object of the invention to devise an arrangement for detecting the rotational speed of a transmission shaft that overcomes the disadvantages cited above. Furthermore, it is an object of the invention to devise a transmission for a motor vehicle with such an advantageous arrangement and, in addition to that, it is the object of the invention to disclose an improved method of detecting the rotational speed of a transmission shaft.